A polarizer used for a polarizing plate is an optical device for converting natural light or any polarized light into polarized light having a certain direction, and has been widely used for a display device such as a liquid crystal display device and an organic light emitting device (OLED). Currently, as a polarizer used for the display device, a polyvinyl alcohol-based polarizing film, in which molecular chains containing an iodine-based compound or a dichroic dye are aligned in a predetermined direction, has been generally used.
The polyvinyl alcohol-based polarizing film is prepared by a method in which a polyvinyl alcohol-based film is dyed with iodine or a dichroic dye and then stretched in a predetermined direction and cross-linked, and in this case, the stretching process may be performed in the form of wet stretching performed in a solution such as a boric acid aqueous solution or an iodine aqueous solution or dry stretching performed under air, and the stretching ratio is generally five times or more.
However, in the preparation process in the related art as described above, it is required that the thickness of an unstretched polyvinyl alcohol-based film exceed 60 μm for the stretching to be performed without causing breakage. The reason is because when the unstretched polyvinyl alcohol-based film has a thickness of 60 μm or less, the degree of swelling of the polyvinyl alcohol-based film is increased, and modulus acting per unit area in the stretching process is increased due to the small thickness, thereby easily causing breakage.
Meanwhile, it is required that the polarizing plate also has a smaller thickness according to the recent trend that display devices have become thin. However, when a polyvinyl alcohol-based film having an unstretched thickness more than 60 μm as in the related art is used, there is a limitation in reducing the thickness of the polarizer. Therefore, studies for preparing a polarizer having a smaller thickness have been made.
For example, Korean Patent Application Laid-Open No. 2010-0071998 discloses a method for preparing a thin polarizing plate using a laminate prepared by coating a hydrophilic polymer layer on a base material layer, or co-extruding a material for forming a base material layer and a material for forming a hydrophilic polymer layer. However, in the case of a coating method or a co-extrusion method, since it is not easy to separate a polyvinyl alcohol layer and a base material layer after stretching and a high peel strength is required for separation, a problem in that a polyvinyl alcohol layer is damaged or deformed during the separation process easily occurs, and as a result, there is a problem in that optical properties such as the degree of polarization of a polyvinyl alcohol film deteriorate, and also, physical properties of a polyvinyl alcohol film prepared according to extrusion conditions, coating conditions or film-forming conditions are easily changed, so that there is a problem in that physical properties of polyvinyl alcohol finally prepared deteriorate and it is difficult to implement uniform physical properties.